Solar cells are used to convert radiant energy into electricity, and can be operated at a relatively low cost as the energy generated is received from the sun.
Typically, a plurality of solar cells are disposed in an array or panel, and a solar energy system typically includes a plurality of such panels. The solar cells in each panel are usually connected in series, and the panels in a given system are also connected in series, with each panel having numerous solar cells. The solar cells in each panel could, alternatively, be arranged in parallel.
Historically, solar power (both in space and terrestrially) has been predominantly provided by silicon solar cells. In the past several years, however, high-volume manufacturing of high-efficiency multi-junction solar cells has enabled the use of this alternative technology for power generation. Compared to Si, multi-junction cells are generally more radiation resistant and have greater energy conversion efficiencies, but they are also heavier (higher density and thickness) and tend to cost more. Some current multi-junction cells have energy efficiencies that exceed 27%, whereas silicon technologies generally reach only about 17% efficiency. When the need for very high power or smaller solar arrays are paramount in a spacecraft or other solar energy system, multi-junction cells are often used instead of, or in hybrid combinations with, Si-based cells to reduce the array size.
Generally speaking, the multi-junction cells are of n-on-p polarity and are composed of InGaP/(In)GaAs III-V compounds. III-V compound semiconductor multi-junction solar cell layers can be grown via metal-organic chemical vapor deposition (MOCVD) on Ge substrates. The use of the Ge substrate has two advantages over III-V compound semiconductor substrates such as GaAs: lower cost and higher structural breakage strength. The solar cell structures can be grown on 100-mm diameter (4 inch) Ge substrates with an average mass density of about 86 mg/cm2. In some processes, the epitaxial layer uniformity across a platter that holds 12 or 13 Ge substrates during the MOCVD growth process is better than 99.5%. Each wafer typically yields two large-area solar cells. The cell areas that are processed for production typically range from 26.6 to 32.4 cm2. The epi-wafers can be processed into complete devices through automated robotic photolithography, metallization, chemical cleaning and etching, antireflection (AR) coating, dicing, and testing processes. The n-& p-contact metallization is typically comprised of predominately Ag with a thin Au cap layer to protect the Ag from oxidation. The AR coating is a dual-layer TiOx/Al2O3 dielectric stack, whose spectral reflectivity characteristics are designed to minimize reflection at the coverglass-interconnect-cell (CIC) or solar cell assembly (SCA) level, as well as, maximizing the end-of-life (EOL) performance of the cells.
In some multi-junction cells, the middle cell is an InGaAs cell as opposed to a GaAs cell. The indium concentration may be in the range of about 1.5% for the InGaAs middle cell. In some implementations, such an arrangement exhibits increased efficiency. The InGaAs layers are substantially perfectly lattice-matched to the Ge substrate.
Regardless of the type of cell used, a known problem with solar energy systems is that individual solar cells can become damaged or shadowed by an obstruction. For example, damage can occur as a result of exposure of a solar cell to harsh environmental conditions. The current-carrying capacity of a panel having one or more damaged or shadowed solar cells is reduced, and the output from other panels in series with that panel reverse biases the damaged or shadowed cells. The voltage across the damaged or shadowed cells thus increases in a reverse polarity until the full output voltage of all of the panels in the series is applied to the damaged or shadowed cells in the panel concerned. This causes the damaged or shadowed cells to breakdown.
As a typical solar cell system has thousands of solar cells, its voltage output is normally in the range of hundreds of volts, and its current output is in the range of tens of amperes. At these output power levels, if the solar cell terminals are not protected, uncontrollable electric discharge in the form of sparks tends to occur, and this can cause damage to the solar cells and to the entire system.
U.S. Pat. No. 6,020,555 describes a solar cell system constituted by panels, each of which includes multiple solar cells, each solar cell being provided with a diode connected between its positive and negative terminals. The provision of the diodes, typically Schottky bypass diodes, does go some way to protecting the solar cells against the uncontrollable electric discharges mentioned above. Unfortunately, however, the air gap left between the terminals of each of the diodes does not eliminate risks of sparking and shorting, which can still occur if moisture or foreign particles bridge the air gap of such a diode. Thus, although air is a dielectric medium, it has a low dielectric strength, which means that, when an electric field across an air gap reaches around 3 mv/m, electric current can jump across the air gap and discharge in the form of sparks. This is referred to as dielectric medium breakdown.
Another shortcoming of the solar cell system described in U.S. Pat. No. 6,020,555 is the inability to manage heat dissipation of the bypass diodes. At a given moment when a solar cell is being “bypassed,” the associated diode (assuming a standard system operating at 600-1000 V, 10 A) will be conducting 6000-10,000 watts of electrical power, some of which is radiated as thermal energy. Given the small size of these diodes, their operational life will be substantially shortened if heat is not well managed. Such a shortcoming is even more of a concern when the solar cell system is, for example, used in connection with a satellite and is, therefore, not field-reparable. Moreover, passive cooling using heat sinks or the like increases weight and is costly both in materials and in fabrication/assembly. Active cooling, while effective at managing the heat generated by the diodes, is very costly and heavy, and expends a substantial amount of the energy that the solar cell system generates.
Another disadvantage of known solar cell receivers is that, owing to the need for such a receiver to generate 10 watts of power at 1000 volts for an extended period of up to, or exceeding, twenty years, there is a danger of sparking at the electrical terminals which connect one receiver of a solar cell system to adjacent receivers.